In digital camera and other imaging applications it is often useful to be able to selectively block light from falling on some portions of an electronic sensor and allow light to fall on other portions of the sensor. Conventional mechanical shuttering techniques have been used for an exposure sequence that sweeps across the sensor exposing only a portion at a time; however, this type of solution can be less desirable for miniaturized sensor arrays due to constraints of size, mechanical complexity, reliability, and cost.
MicroElectroMechanical Systems (MEMS) technology has been employed for providing shutter arrays with selectively actuable shutters in a number of display applications. For example, MEMS shutter arrays for display devices are described in U.S. Pat. No. 5,078,479 entitled “Light Modulation Device with Matrix Addressing” issued to Vuilleumier and U.S. Pat. No. 5,784,190 entitled “Electro-Micro-Mechanical Shutters on Transparent Substrates” issued to Worley. With display devices of this type, MEMS shutters correspond to image pixels, so that each display pixel is formed using one or more shutter elements.
MEMS shutter arrays have also been adopted for use in light sensor applications. In one highly publicized NASA program, a MEMS microshutter array has been under development to support field selection for the near infrared spectrograph on the James Webb Space Telescope (JWST). This array uses a 128×64 element matrix of individually actuable 100 micron×200 micron shutter blades for variable field selection. Each shutter blocks light to a separate cell, with an electrode along one of the cell walls. Each shutter is normally closed until actuated. The shutter blades themselves are electromagnetically opened by sweeping a magnet over the array, then electrostatically latched over the desired target positions. This device is described in a paper entitled “Microshutter Array Development for the James Webb Space Telescope” by Li et al. in Proceedings of SPIE, Micro- and Nanotechnology: Materials, Processes, Packaging, and Systems II, volume 5650, pp. 9-16.
Recognizing inherent limitations of the solution used for the JWST device, astronomy researchers working with the Japan Aerospace Exploration Agency have proposed an alternate microshutter array that is electrostatically actuated. Their work is described in a paper entitled “Development of Microshutter Arrays for Ground-Based Instruments” by Motohara et al. presented at the Workshop for Instrumentation on Extremely Large Telescopes, Ringberg, Germany, July, 2005. The Motohara et al. device similarly uses an array of cells with a single shutter for each cell, normally closed until actuated and with its actuating electrode along a cell wall.
While MEMS microshutters have been developed for various display devices and for astronomical instrumentation, however, the solutions that have been proposed for either the JWST or Motohara et al. devices would be difficult to scale to the dimensions required for digital camera applications or for hand-held image capture devices in general. Moreover, even if such devices could be scaled to the dimensions needed for digital camera applications, problems such as poor fill factor, inherent fabrication difficulties, and disappointing shutter response times would significantly compromise their usefulness. These devices require considerable surface area for actuation and mechanical support components, severely reducing the proportion of the array that is available for transmission of light. This problem is particularly troublesome for small digital image capture apparatus, where it is important to provide as much light as possible from the object field.
Operational requirements further compound the problem. For example, the electromechanical initialization used in the JWST device, sweeping a magnet over the microshutter array in order to initially latch shutters in closed position, is impractical for implementation in a digital camera or other digital imaging apparatus. The high latching voltages required by the Motohara et al. device, in excess of 100 Vdc, render this type of solution wholly unsuitable for use within digital cameras. Both the JWST and Motohara et al. devices use electrodes that lie along a cell wall, which would not be an arrangement that could be readily fabricated at a smaller size using known MEMS techniques.
Thus, although MEMS microshutter arrays have been developed for some types of display and image-sensing applications, e.g., large separate cell arrays, there remains a need for a microshutter array solution that is suitable for use with digital cameras and other hand-held imaging devices.